1. Field of the Invention
The present invention relates to polymer fine particles and an ink composition containing the same. In particular, the present invention relates to polymer fine particles and an ink composition containing the same, wherein the polymer fine particles are excellent in dispersion stability, and they stably carry a dye or a pigment in the ink composition.
2. Description of the Related Art
Polymer fine particles attract attention in various fields such as those concerning paints and inks, since polymer fine particles can be used as, for example, a cosmetic additive, a plastic modifier, and a viscosity-adjusting agent. For example, Japanese Patent Publication No. 60-42833 discloses an ink for ink-jet printers, containing polymer fine particles prepared by emulsion-polymerizing a vinyl monomer, the polymer fine particles being impregnated with or adsorbed to a dye. Usually, as for such polymer fine particles, in order to improve the dispersion stability of the particles in a medium, surfaces of the polymer fine particles are modified with electrically charged groups. The polymer fine particles disclosed in the foregoing patent document have their surfaces modified with sulfonic groups such as --SO.sub.3 Na. On the other hand, Japanese Patent Laid-open No. 5-163700 discloses a paint composition containing emulsion particles made of a polymer together with a pigment. In this technique, polymer fine particles are also modified with sulfonate groups or the like.
However, the sulfonate group involves a problem that the dye or the pigment in the ink or the paint composition is deteriorated because the sulfonate group is a strong acid polar group. Especially, when an ionic dye is contained in the paint, the dye is markedly deteriorated. Further, when the ink composition or the paint containing such polymer fine particles is used in a machine such as a printer having metallic parts, a problem arises in that halide ion corrodes the metallic parts such as a printer nozzle.
On the other hand, polymer fine particles, which have surfaces modified with ammonium salt groups, are known (for example, C. Wu, Macromolecules, 27, 7099 (1994)). Such polymer fine particles do not cause the problem of deterioration of the dye because the particles have weakly basic surfaces. However, usually, the counterion of the ammonium base is principally halide ion represented by chloride ion. Therefore, when an ink composition containing such polymer fine particles is used in a printer, a problem arises in that halide ion corrodes metallic parts such as a printer nozzle.